The invention relates to a method of determining the paper quality for autotypical halftone printing, in which a finely distributed pattern of geometric figures is applied to the paper, the paper is illuminated and the light reflected and scattered by the paper is observed.
During printing, the problem occurs that gray tones can be reproduced only poorly. In order nevertheless to be able to produce prints of good quality with graduated lightness tones, use is therefore made to a widespread extent of the technique of autotypical halftone printing, as it is known. In this case, the image is no longer determined by continuous lightness transition. Instead, the image is built up from a finely distributed pattern of geometric figures, which are normally dots. In a widespread printing technique, these dots are arranged regularly in a grid. Depending on the lightness value at a specific point of the image to be printed, these dots are made larger or smaller. If the points are arranged sufficiently densely and if the printed image is viewed from a sufficient distance, then the individual dots are no longer perceived, but the impression is given of an image with different gray tones or, if dots of different colors are used, of various hues. In certain applications, the dots can also be arranged irregularly, the spacing or the number of dots determining which gray tones or hues are perceived by the observer.
The size of such a dot can be calculated in accordance with the lightness of the image point. At the same time, it is necessary to take account of the fact that the dot will in practice assume a greater area or the printed image perceived by the eye will be more influenced than would have to be the case in theory. One problem in printing technology, which depends on the paper quality of the printing technology and other factors, consists in the fact that, during printing, the dot becomes physically larger than is intended. In particular in the case of prints of high quality, it is of course necessary to take this into account. For such prints, use is normally made of coated paper in which, by means of appropriate surface configuration, it is ensured that the dot does not become significantly larger during printing than is intended. Here, another problem of the apparent enlargement of the dot occurs. The coating is not completely opaque. Instead, the light penetrates into the coating down to a certain depth and into the color layer lying underneath and is then scattered back. The light scattered back does not always reach the eye of the observer, however. If a light beam penetrates under an oblique angle into the coating very close to the edge of a dot, it can be scattered under the dot and no longer emerge from the paper. Light beams which enter close to the dot therefore to some extent do not emerge from the coating again, so that the dot appears larger than it actually is; the edge becomes “blurred”, and exhibits a shadow (which is referred to as halation).
There are mathematical models relating to how this dot gain can be determined (Dissertation “Dot Gain in Colour Halftones” by Stefan Gustavson, Linköping, September 1997). This actual or apparent enlargement of the dots can be taken into account on the basis of this mathematical model or else on the basis of optical observation of the finished print, so that theoretically satisfactory prints can be produced. However, this is a purely theoretical conclusion. This cannot be implemented technically, since no printed dots matched appropriately to the respective location can be produced.
However, it has been shown that the optical surface impression of a coated paper is not completely uniform. For example, the paper can have a nonuniform gloss which, of course, leads to a nonuniform intensity of the reflected or scattered light when the print is viewed. The lightness or white color of the coating can be different, which can be caused by a certain blackening, visible fibers of the paper which are not sufficiently covered by the coating and/or a nonuniform distribution of optical whitening agents. The hiding power or opacity of the coating can also be nonuniform, which can be related with the preceding causes, so that light beams penetrate into the paper structure to different depths. In addition, the thickness of the coating can vary locally, which leads to different scatter in the coating and also to nonuniform enlargement of the perceived image of the dot.
All these imperfections of the paper lead to different dot gains. It will be possible to detect these to a more or less greater extent when the print is observed, when the latter has been finished. This is where the present invention starts, it being unsatisfactory, of course, for the deficient quality of the paper to be detected only after the complicated production of the print. In particular, of course a paper manufacturer cannot apply such prints intermediately for test purposes in the reels of paper supplied by him.
The object of the invention consists in providing a method of the type mentioned at the beginning with which the quality of the paper can be determined simply and reliably before printing and without such a print.